Increment the I_P_List counter whenever a element is inserted into

the list. Previously, the counter would only be incremented if the
insertion method push_front() was used, in which case the counter
wouldn't be incremented if a element was inserted using the push_back()
and/or insert_after() methods.

Currently this does not affect the code base because there isn't any
code that uses a counted list with the push_back() or insert_after()
methods.

sql/sql_plist.h

@@ -95,6 +95,7 @@ class I_P_List : public C, public I
     *last= a;
     *B::prev_ptr(a)= last;
     I::set_last(B::next_ptr(a));
+    C::inc();
   }
   inline void insert_after(T *pos, T *a)
   {
@@ -112,6 +113,7 @@ class I_P_List : public C, public I
       }
       else
         I::set_last(B::next_ptr(a));
+      C::inc();
     }
   }
   inline void remove(T *a)